Conventional facsimile (fax) machines are commonly used to transmit copies of documents over the public switched telephone network (PSTN). Such facsimile machines generally connect to one another based on a protocol known as the T.30 protocol.
More recently, packet-based networks, such as Internet Protocol (IP) networks, are being used as alternatives to conventional PSTN networks. Voice over IP (VoIP), for example, is commonly used to transmit voice over IP networks. However, because most VoIP systems are optimized (e.g., through the use of bandwidth-saving compression) for voice rather than data calls, conventional facsimile machines may work poorly or not at all. The T.38 fax relay standard is one protocol designed to address this problem.
In a typical combined T.38 and T.30 connection, analog fax terminal devices communicate over a PSTN network using the T.30 protocol with gateway devices. The gateway devices communicate over IP networks using the T.38 protocol. In general, the T.38 protocol functions to make the IP network “invisible” to the two endpoint analog fax terminals. The T.38 protocol may, for example, correct for network delays with so-called spoofing techniques or other techniques so that the two fax terminals are “fooled” into “thinking” that they are each communicating directly with one another.